Piston pump for a brake system for a vehicle

ABSTRACT

A piston pump for a hydraulic slip-controlled brake system for a vehicle. To seal off and guide a piston of the piston pump in a bush, the piston pump includes a seal of rigid plastic which has a spreader face in the form of an internal cone. A piston restoring spring presses directly or indirectly against the spreader face via a spreader ring with a conical counterpart face and thereby presses the seal into sealing contact with the bush. The piston pump has the advantage that no separate sealing ring or guide ring is needed for the piston, and that friction and wear are both only slight.

PRIOR ART

[0001] The invention is based on a piston pump as generically defined by the preamble to the main claim, which is intended in particular for use in a hydraulic vehicle brake system with slip control.

[0002] Such piston pumps are known per se. They have a piston that is received axially displaceably in a cylinder bore of a pump housing. A bush can be inserted into the pump housing, or the piston can be received directly in the pump housing. By means of an eccentric drive mechanism, the piston can be driven to execute an axially reciprocating stroke motion. In the known piston pumps, for sealing between the piston and the cylinder bore, a sealing ring of soft, rubberlike plastic, often an O-ring, is inserted into a piston groove or a groove in the cylinder bore. It is also known to guide the piston with a guide ring in the cylinder bore; the guide ring is inserted together with the sealing ring into the piston groove or the cylinder bore groove. The guide ring, which typically comprises a dimensionally stable plastic, has the advantage that it lessens friction and wear on the piston and the cylinder bore wall. Another advantage of a guide ring is that the piston and/or the cylinder bore can be made with greater diameter tolerance and a lesser surface quality.

[0003] In the seal, it is important that adequate elasticity always be present, so that the seal will always be prestressed forcefully enough against the slide face regardless of dimensional and shape tolerances and regardless of wear.

[0004] Since at high pressures, only very small gaps can be spanned by seals made of a material with adequate intrinsic elasticity, very close dimensional and shape tolerances must be met overall, which makes the effort and expense of producing the piston pump quite high. Often, a support ring is also required, to assure that the seal will not be squeezed into the gap. The support ring, too, increases the production effort and expense considerably.

[0005] The known piston pumps have the disadvantage that besides the sealing ring, an additional support ring and/or an additional guide ring of plastic is also required. Piston pumps without the guide ring have the disadvantage of major piston friction, greater wear, and closer production tolerances for the diameter of the piston and of the cylinder bore, and of requiring relatively high surface quality on the piston circumference and the cylinder bore surface, which typically means that these surfaces have to be ground or honed.

ADVANTAGES OF THE INVENTION

[0006] The piston pump of the invention having the characteristics claim 1 has the advantage that because of the spring element for the seal, which prestresses the seal against the slide face, a relatively rigid material, with little intrinsic elasticity but in return with especially low friction and low wear, can also be used for the seal. Because a very rigid and relatively inelastic material can be chosen for the seal, the advantage is attained that even at high pressures, even relatively large gap dimensions can be permitted. Thus even without using a separate support ring, relatively wide dimensional and shape tolerances can be allowed, which advantageously reduces the effort and expense of production.

[0007] Because it is possible to use a material with low friction and consequently low piston friction and little wear, the advantage is attained that the pumping capacity is improved and the useful life of the piston pump of the invention is prolonged, without requiring a separate guide ring. The seal is elastically prestressed radially against a slide face by the spring element. Depending on the mode of embodiment, the seal is either elastically widened in the radial direction by the spring element and thus is pressed sealingly against a cylinder bore wall, when the seal is mounted axially nondisplaceably on the piston and is axially displaceable together with the piston in the cylinder bore, or else the seal is elastically radially compressed by the spring element, so that the seal, when it is axially nondisplaceably mounted in the cylinder bore and the piston in its stroke motion is axially displaced relative to the seal, presses sealingly against a piston circumference. By using the spring element that presses the seal into sealing contact with the slide face, it is possible to use a seal of a relatively rigid plastic, that is, a material from which guide rings are already made, and which has low friction and low wear. A relatively soft, rubberlike plastic of the kind usually used for sealing rings is not suitable for sealing purposes, because given its lack of elasticity, or its inadequate elasticity, it does not assure adequate sealing, at least over the long term. At an elevated temperature or upon a temperature change, if not before, a guide ring made of rigid plastic loses its sealing effect. This problem is overcome with the spring element used according to the invention.

[0008] The spring element assures that under all operating conditions, the seal will be prestressed sufficiently elastically against the slide face. As a result, the advantage is attained of good sealing under all operating conditions, even if the hydraulic pressure is low and also even if the seal is of a relatively rigid material. Hence the piston pump is advantageously also suitable even for widely varying operating conditions; at low hydraulic pressure, the spring element assures adequate prestressing of the seal against the slide face, and for high hydraulic pressure, the material of the seal can be selected with adequate rigidity.

[0009] Advantageous features and refinements of the invention defined by the main claim are the subject of the dependent claims.

[0010] In a piston pump, a piston restoring spring, typically provided in a high-pressure work chamber of the piston pump and acting axially, presses the piston of the piston pump into constant contact with an eccentric element. This piston restoring spring, acting in the axial direction, can take on the function of the spring element that urges the seal in the radial direction. By a suitable choice of the inclination of the spreader face, the desired radially acting force can be adapted very easily to the axially acting force generated by the piston restoring spring. Because the piston restoring spring can take on the task of the spring element, the advantage is attained that only a few components in all are needed. Because the force of the piston restoring spring typically does not change over the entire useful life of the piston pump, the force urging the seal in the radial direction advantageously also stays constant, regardless of any possible wear between the seal and the slide face. As the preferred exemplary embodiments show, no further spring element except for the piston restoring spring, which is typically present anyway, is needed to furnish the force that urges the seal against the slide face.

[0011] When the piston of the piston pump is completely retracted, the pressure in the high-pressure work chamber of the piston pump is typically at its highest, while when the piston is completely extended, the pressure in the high-pressure work chamber is typically at its lowest. Accordingly, since the force generated by the piston restoring spring is greatest when the pressure in the high-pressure work chamber is also greatest, the advantage is attained of good sealing action, even at high pressure, and of less wear at low pressure.

[0012] In one feature of the invention, the seal has a spreader face, for instance a conical annular face, that is inclined relative to a radial direction, and this face is urged directly or indirectly by a spring element, such as a helical compression spring, with a force acting in the axial direction. Because the inclination of the spreader face is oriented radially inward or outward, the axial force of the spring element is converted into a radial force, which radially widens or compresses the seal and thus keeps it in sealing contact with the slide face, that is, on the cylinder bore wall or on the piston circumference, depending on the embodiment. The inclination of the spreader face, which can vary in the radial direction of the spreader face, determines the ratio by which the axial force of the spring element is converted into a radial force that presses the seal sealingly against the slide face, so that the radial force exerted on the seal can be varied purposefully by the choice of the inclination of the spreader face.

[0013] The piston pump of the invention is intended in particular as a pump in a brake system of a vehicle and is used to control the pressure in wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS, ASR, FDR and EHB are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from a wheel brake cylinder or a plurality of wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into a wheel brake cylinder or a plurality of wheel brake cylinders (ASR or FDR or EHB). In a brake system with wheel slip control (ABS or ASR) and/or a brake system serving as a steering aid (FDR) and/or an electrohydraulic brake system (EHB), the pump is needed. With the wheel slip control (ABS or ASR), locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (ASR) can for instance be prevented. In a brake system serving as a steering aid (FDR), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from breaking out of the track desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system.

DRAWING

[0014] The invention will be described below in further detail in terms of selected exemplary embodiments shown in the drawing. The three drawing figures show axial sections through three piston pumps of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0015] The piston pump of the invention, shown in FIG. 1 and identified overall by reference numeral 10, has a bush 12, which is inserted into a stepped pump bore 14 of a hydraulic block that forms a pump housing 16. The hydraulic block, of which the drawing shows only a fraction surrounding the piston pump 10, is part of a slip-controlled hydraulic vehicle brake system not otherwise shown. Besides the piston pump 10, other hydraulic components such as magnet valves and hydraulic reservoirs are inserted in the hydraulic block, and a master cylinder and wheel brake cylinders are connected to it. By means of the hydraulic block, the hydraulic components are connected hydraulically to one another.

[0016] A boltlike piston 20 is received in the bush 12 and protrudes for a short distance from the bush 12 on one end. The end of the piston 20 protruding from the bush 12 is guided axially displaceably by means of a guide ring 22 in the pump bore 14 in the pump housing 16 and is sealed off in the pump housing 16 by an O-ring 24. For fluid admission, the piston 20 is provided with an axial blind bore 26 made from its end located in the bush 12; the blind bore is intersected, near its bottom, by transverse bores 28. A rated diameter of the piston 20 is equivalent to an inside diameter of the bush 12, with clearance between the piston 20 and bush 12; that is, relative to the bush 12, the piston 20 has an undersize that assures the axial displaceability of the piston 20. The underside of the piston 20 creates a gap all the way around between the piston 20 and the bush 12. To assure that the piston 20 will not touch the bush 12 even if dimensional and shape tolerances are unfavorable, the gap must be made large enough. The blind bore 26 and the transverse bores 28 communicate, through a wide groove 30 in the circumference of the piston 20 and radial bores 32 in the bush 12, with an inlet bore 34 which is made in the pump housing 16 radially to the piston pump and discharges into the pump bore 14 into which the bush 12 is inserted.

[0017] As its inlet valve 36, the piston pump 10 of the invention has a spring-loaded check valve, which is mounted on the end of the piston 20 that is located in the bush 12. An orifice of the blind bore 26 is embodied as a conical valve seat 38, against which a valve ball 40, as a valve closing body, is pressed by a helical compression spring acting as a valve closing spring 42. The valve closing spring 42 is braced against the bottom of a cup-shaped valve cage 44, which is made of sheet metal as a deep-drawn part and has openings 46. On its open side, the valve cage 44 has an annular shoulder 48, extending all the way around, with which it rests on the face end of the piston 20 that is located in the bush 12. A free edge 50 of the valve cage 44 that is deformed inward engages a groove, punched into the piston 20, in the manner of a clip connection, and as a result the valve cage 44 is secured to the piston 20. The valve ball 40 and the valve closing spring 42 are received in the valve cage 44.

[0018] For driving the piston 20 to execute an axially reciprocating motion, the piston pump 10 of the invention has an eccentric element 52, which can be driven to rotate by an electric motor and against whose circumference the piston 20 is pressed by a piston restoring spring 54, which is embodied as a helical compression spring and is disposed in the bush 12 and braced on the bush bottom 18.

[0019] A cylindrical closure part 56 is mounted on the bush bottom 18 and is joined to the bush 12 with a crimp 58. By the caulking 59 of the pump housing 16, the closure part 56 closes the pump bore 14 in pressure-tight fashion and fixes the bush 12 in the pump housing 16. An outlet valve 60 in the form of a spring-loaded check valve is accommodated in the closure part 56: The closure part 56 has a coaxial blind bore 52, into which a helical compression spring, as the valve closing spring 64, and a valve ball 66, as the valve closing spring, are inserted. The valve ball 66 cooperates with a conical valve seat 68, which is mounted at an orifice of a center bore 70 that passes axially through the bush bottom 18. The valve seat 68 is shaped and hardened by swaging. A discharge of the fluid is effected through radial conduits 72 between the bush bottom 18 and the valve seat 56 into an outlet bore 74 in the pump housing 16.

[0020] The available volume in the region of the groove 30 and the radial bore 32 serves as a low-pressure chamber 75 for the piston pump 10. Between the inlet valve 36 and the outlet valve 60, there is a high-pressure work chamber 77 of the piston pump 10. In accordance with the outward and inward motions of the piston 20, the volume of the high-pressure work chamber 77 increases and decreases, respectively. For sealing off the high-pressure work chamber 77 from the low-pressure chamber 75, the piston 20 has a seal 76, which extends annularly all the way around and is placed on the end of the piston 20 located in the bush 12, on an annular shoulder 78 where the piston 20 narrows toward its end located inside the bush 12. The seal 76 is of relatively rigid plastic, preferably PTFE (polytetrafluoroethylene), which has a low coefficient of friction. The seal 76 also serves to guide the piston 20 in the bush 12. In the exemplary embodiment selected, the seal 76 can therefore be called a sealing and guide ring. Because the seal 76 can be made from relatively rigid, dimensionally stable plastic, it can span even a relatively wide gap between the piston 20 and the bush 12. Thus good sealing and durability are assured even at high pressures. To prevent the seal from being forced into the gap at high pressures, until now a support ring was often provided. In the piston pump 10 proposed here, no such support ring is necessary.

[0021] The seal 76 shown as an example is virtually rectangular in annular cross section; an end face toward the bush bottom 18 is conical and acts as a spreader face 80, or more precisely forms an internal cone; that is, imaginary straight lines perpendicular to the spreader face 80 run obliquely inward. The piston restoring spring 54 presses axially against the seal 76, via a ring 82 placed between it and the seal 76, and presses the piston into contact with the circumference of the eccentric element 52, via the annular shoulder 78 of the piston 20. The ring 82 acts as a spreader ring. The ring 82 is embodied as a perforated conical disk, with the same cone angle as the spreader face 80. A face end of the ring 82, contacting the spreader face 80 of the seal 76, forms a counterpart face 84 to the spreader face 80. Via the ring 82 with the conical counterpart face 84 that contacts the conical spreader face 80 of the seal 76, the piston restoring spring 54 presses the seal 76 radially spreadingly into sealing contact with the bush 12. In the preferably selected exemplary embodiment, the inner circumference of the bush 12 serves as a slide face 85. In the stroke motions of the piston 20, the seal 76 slides on the slide face 85. An angle of inclination of the counterpart face 84 and the spreader face 80 to a radial direction, together with the friction between the ring 82 and the seal 76, determines the contact pressure of the seal 76 against the bush 12 in proportion to the spring force of the piston restoring spring 54. The rigidity of the seal 76, which depends on the rigidity of the material and on the cross-sectional area of the seal 76, also affects the force with which the seal 76 is pressed against the slide face 85. To obtain a sufficiently strong radial force pressing the seal 76 against the slide face 85 even if a rigid, low-wear material is used for the seal 76, the spreader face 80 can be inclined correspondingly more markedly relative to the radial direction. The piston restoring spring 54 at the same time forms a spring element that spreads the seal 76 radially, and thereby assures the sealing with respect to the bush 12. With respect to the piston 20, the seal 76 provides sealing by means of contact with the annular shoulder 78 of the piston 20.

[0022] The counterpart face 84 and the spreader face 80 need not have the same inclination to the radial direction, and in particular the counterpart face and/or the spreader face can be spherical or hollow and round, for instance. The ring 82 acting as a spreader ring can also be used with a circularly round or semicircular annular cross section (not shown), for instance. Also, the piston restoring spring 54 can press against the spreader face 80 of the seal 76 without any ring being disposed between them.

[0023] In the exemplary embodiment of the invention shown in FIG. 2, the same kind of seal 76 is used as in the piston pump 10 shown in FIG. 1. The conical spreader face 80 of the seal 76, however, faces toward the annular shoulder 78 of the piston 20, which is embodied conically as a counterpart face 88 for the spreader face 80. Via the ring 82 disposed between them, the piston restoring spring 54 presses the seal 76, extending annularly all the way around, against the inclined counterpart face 88, thereby pressing the seal 76 in radially spreading fashion into sealing contact with the slide face 85 provided on the bush 12. The ring 82 assures that the seal 76 will not be damaged by the piston restoring spring 54. Since a fairly rigid material can be used for the seal 76, it is optionally possible to omit the ring 82, without damage to the seal 76 from the direct contact of the piston restoring spring 54 with the seal 76.

[0024] Otherwise, the lp 10 shown in FIG. 2 matches the piston pump 10 shown in FIG. 1. To avoid repetition, reference is made to the corresponding description of FIG. 1. Identical components are identified by the same reference numerals.

[0025] In the first exemplary embodiment (FIG. 1), the face end of the seal 76 toward the ring 82 or toward the piston restoring spring 54 is conically inclined relative to the radial direction. In the second exemplary embodiment (FIG. 2), both the face end of the seal 76 remote from the piston restoring spring 54 and the annular shoulder 78 of the piston 20 are conically inclined relative to the radial direction. The piston pump 10 can also be modified in such a way that both the face end of the seal 76 toward the ring 82 or toward the piston restoring spring 54 and the face end of the seal 76 toward the annular shoulder 78 are conically inclined. In that case, the direction of inclination of the face end toward the ring 82 or toward the piston restoring spring 54 extends as shown in FIG. 1, while the direction of inclination of the face end of the seal 76 toward the annular shoulder 78 is as shown in FIG. 2 In that case, both face ends of the seal 76 act as a spreader face, and as a result the inclination of the two spreader faces relative to the radial direction can be relatively slight. Thus even if the two face ends of the seal 76 are only slightly inclined, a relatively major action pressing the seal 76 against the slide face 85 is attainable.

[0026] In the exemplary embodiments shown in FIGS. 1 and 2 the piston 20 substantially comprises metal, and the seal 76, which can also act as a guide for the piston 20 in the bush 12 or directly in the pump housing 16, is slipped as a separate component onto the piston 20. However, it is also possible to make the piston 20 partly or entirely of plastic, and it is furthermore possible to form the seal 76 integrally onto the piston 20, or onto the part of the piston 20 that is plastic. FIG. 3 shows an exemplary embodiment in which the piston 20 substantially comprises a first piston part 20 a and a second piston part 20 b. A sealing composition (not shown) that may optionally be applied between the two piston parts 20 a, 20 b assures good sealing between these two parts. The first piston part 20 a toward the eccentric element 52 is of metal, and the second pump part 20 b is of plastic, preferably a rigid, dimensionally stable plastic. In this exemplary embodiment, the seal 76 is formed integrally onto the piston part 20 b of the piston 20. The face end of the seal 76 toward the piston restoring spring 54 or toward the ring 82 is embodied conically or in tapered fashion. The spreader face 80 is located on this face end. As FIG. 3, shows, the inclination of the face of the ring 82 that contacts the spreader face 80 of the seal 76 is adapted to the inclination of the spreader face 80. The piston restoring spring 54 acts in the axial direction on the piston 20, and because of the inclination of the spreader face 80, the force of the piston restoring spring 54 acting axially on the piston 20 engenders a force component that acts radially outward on the seal 76. In the exemplary embodiment shown in FIG. 3, essentially only the region of the piston part 20 b acted upon directly by the piston restoring spring 54 is urged radially outward against the slide face 85, so that this region of the piston 20 takes on the function of the seal 76 and performs it excellently, even if the piston 20 or the piston part 20 b is or a fairly rigid material that has little elasticity of its own. Because the region of the piston 20 oriented toward the high-pressure work chamber 77 is prestressed elastically outward in the radial direction toward the slide face 85, excellent sealing of the high-pressure work chamber 77 from the low-pressure chamber 75 is obtained. In this exemplary embodiment again, the advantage is gained that for sealing between the high-pressure work chamber 77 and the low-pressure chamber 75 and for the guidance of the piston 20 inside the bush 12 or inside the pump housing 16, two separate components are not needed. Once again, even at high pressures, no separate support ring to protect the seal 76 is needed.

[0027] In the piston pumps 10 of the invention as shown in the drawing, instead of being sealed and guided by a separate sealing ring 24 and guide ring 22, as is the end (not shown) of the piston 20 that is located in the bush 12, the end of the piston 20 that protrudes from the bush 12 can be sealed off in the same way with a plastic seal (not shown) that is held in sealing contact with the slide face by a spring element. This variant embodiment that is not shown has a seal that is fixedly associated with the pump housing 16 and is urged radially against the slide face, provided on an outer circumference of the piston, by the spring element. 

1. A piston pump, having a piston that can be driven to execute a reciprocating stroke motion and is axially displaceably guided in a cylinder bore, and having a seal which seals at a slide face, characterized in that the seal (76) is urged radially against the slide face by a spring element (54).
 2. The piston pump according to claim 1 , characterized in that the piston pump (10) has a piston restoring spring (54), which simultaneously forms the spring element.
 3. The piston pump according to claim 1 or 2 , characterized in that the seal (76) serves to guide the piston (20).
 4. The piston pump according to one of claims 1-3, characterized in that the seal (76) has a spreader face (80) with an inclination to a radial direction, and that the spring element (54) urges the spreader face (80) in the axial direction, so that the seal (76) is urged in the radial direction.
 5. The piston pump according to one of claims 1-4, characterized in that the piston pump (10) has a counterpart face (84, 88) to the spreader face (80) of the seal (76), and the counterpart face (84, 88) is oriented toward the spreader face (80) and has an inclination to a radial direction that is oriented opposite the inclination of the spreader face (80), and that the seal (76) is pressed with its spreader face (80) against the counterpart face (84, 88) by the spring element (54), so that the seal (76) is urged in the radial direction.
 6. The piston pump according to claim 5 , characterized in that the counterpart face (84) is formed on a ring (82).
 7. The piston pump according to claim 5 , characterized in that the counterpart face (88) is formed on the piston (20).
 8. The piston pump according to claim 5 , characterized in that the counterpart face (88) is formed in the cylinder bore of the pump housing (16).
 9. The piston pump according to claim 1 , characterized in that the seal (76) is a plastic ring.
 10. The piston pump according to one of the foregoing claims, characterized in that the seal (76) is integrally formed onto the piston (20).
 11. The piston pump according to claim 9 , characterized in that the seal (76) comprises PTFE (polytetrafluoroethylene). 